Optical device and manufacturing method therefor

ABSTRACT

An optical device includes, in sequence, a surface formed of a metal oxide, a samarium oxide-containing layer in contact with the surface formed of a metal oxide, and a magnesium fluoride-containing layer in contact with the samarium oxide-containing layer so as to suppress optical absorption resulting from high-rate sputter deposition of a magnesium fluoride-containing layer on a surface formed of a metal oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/854,619, filed Apr. 21, 2020, which claims the benefit of JapanesePatent Application No. 2019-085961, filed Apr. 26, 2019, each of whichis hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an optical device and a manufacturingmethod therefor.

Description of the Related Art

There is a known structure in which a fluoride layer, such as analuminum fluoride (AlF₃) layer or a magnesium fluoride (MgF₂) layer, isformed on a surface of an optical device, such as a lens or a mirror, toserve as an antireflection layer.

A vacuum evaporation method offers advantages such as simple devicestructure and high film deposition rate, and thus antireflection layershave been formed mostly by the vacuum evaporation method. However, inrecent years, there has been increasing demand for film deposition by asputtering method that is superior to the vacuum evaporation method fromthe viewpoints of reproducibility, film variation control,low-temperature deposition, etc.

A sputtering method is a method of forming films by sputtering amaterial in an atomic state using charged particles, such as a plasma.In film deposition by a sputtering method, it is difficult to controlthe reaction between the sputter material and a substrate material andto control the damage inflicted by the charged particles on thesubstrate and the film. For example, when a film containing a fluorideis formed on a surface of a metal oxide by a sputtering method, themetal oxide is reduced by fluorine, and absorption occurs on the longerwavelength side with respect to the wavelength corresponding to the bandgap. As a result, performance of the optical device is degraded.

Japanese Patent Laid-Open No. 9-291358 discloses a method for forming anoxide layer between a glass substrate and a fluoride layer (MgF₂ layer),the oxide layer containing at least one of SiO₂, ZrO₂, and Al₂O₃, inorder to suppress optical absorption that occurs at the interfacebetween the glass substrate and the fluoride layer. Japanese PatentLaid-Open No. 2015-114599 discloses a method for forming, as a base fora fluoride layer (MgF₂ layer), a magnesium oxyfluoride layer representedby Mg_(x)O_(y)F_(z) (where 0.01≤z/x≤1.45 and 0.01≤z/y≤3.17).

At the site of production, in order to increase productivity, it isdesirable to increase the density of the plasma near the substrate andspeed up film deposition by increasing the input power and decreasingthe distance between the target and the substrate. However, when afluoride layer is deposited at a high speed by the methods described inthe aforementioned patent documents, separation and optical absorptionoccur at the interface between the fluoride layer and the substrate. Theoptical absorption that occurs at the interface is considered to beattributable to local breaking of atomic bonds caused by irradiation ofthe interface with electrons in the plasma and accelerated high-energycharged particle beams. As described above, the known art rarelyprovides optical devices with satisfactory characteristics by high-ratefilm deposition.

SUMMARY OF THE INVENTION

The present disclosure provides an optical device that can suppressoptical absorption and that has high optical transmittance in a visiblelight region, by forming a magnesium fluoride-containing layer on asurface of a metal oxide by high-rate sputter deposition.

An aspect of the present disclosure provides an optical device thatincludes, in sequence, a surface formed of a metal oxide; a samariumoxide-containing layer in contact with the surface formed of a metaloxide; and a magnesium fluoride-containing layer in contact with thesamarium oxide-containing layer.

Another aspect of the present disclosure provides an optical devicemanufacturing method that includes, in sequence, a step of forming asamarium oxide layer or an ytterbium oxide layer on a base member havinga surface formed of a metal oxide by reactive sputtering using a metaltarget formed of samarium or ytterbium; and a step of forming amagnesium fluoride layer by reactive sputtering using a metal targetformed of magnesium.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an optical device according toan embodiment of the present disclosure.

FIG. 2 is a diagram illustrating an example of a deposition systemsuitable for realizing an optical device f the present disclosure.

FIG. 3 is a graph in which some binding energy region in a Surveyspectrum of an Ar2p3/2 peak of a Sm₂O₃ layer is extracted for a samplein which a H₂ gas was introduced during deposition and a sample in whichH₂ gas was introduced at a rate of 100 sccm.

FIG. 4 indicates a narrow spectrum in the depth direction of a Sm3d5/2peak around the interface between a Sm₂O₃ layer and a MgF₂ layer when aH₂ gas is introduced at a rate of 100 sccm during deposition of theSm₂O₃ layer.

FIG. 5 indicates a narrow spectrum in the depth direction of the Sm3d5/2peak around the interface between the Sm₂O₃ layer and the MgF₂ layerwhen a H₂ gas is not introduced during deposition of the Sm₂O₃ layer.

DESCRIPTION OF THE EMBODIMENTS

An optical device and a manufacturing method therefor according to thepresent disclosure will now be described with reference to the drawings.Although examples in which the gas types and the film materials arelimited are described in this embodiment, the scope of the presentinvention is not limited by these examples.

Optical Device

FIG. 1 is a schematic sectional view of an optical device according tothe present disclosure. An optical device 100 includes, in sequence, abase member 120 constituted by a substrate 101 and a layered structure111 disposed on the substrate 101, a buffer layer 112, and a magnesiumfluoride-containing layer 113.

Calcium fluoride crystals, quartz glass, silicon, glass, a resin, or thelike can be used as the substrate 101. The substrate 101 can havevarious shapes depending on the usage of the optical device, and mayhave a flat shape as illustrated in FIG. 1 , or a curved surface.

The layered structure 111 includes multiple layers of metal oxidesstacked on top of each other. The layered structure 111 is obtained byalternately stacking layers 111 a having relatively high refractiveindices (high-refractive-index layers) and layers 111 b havingrelatively low refractive indices (low-refractive-index layers), and isformed to prevent reflection at the surface of the substrate 101.Specifically, the high-refractive-index layers 111 a are metal oxidelayers having a refractive index of 1.75 or more and 2.70 or less, andcontain zirconium oxide, tantalum oxide, niobium oxide, titanium oxide,or the like. The low-refractive-index layers 111 b are metal oxidelayers having a refractive index of 1.35 or more and 1.75 or less, andcontain silicon oxide, aluminum oxide, or the like. The materials andlayer thickness for the high-refractive-index layers 111 a and thelow-refractive-index layers 111 b may be determined according to theoptical design of the optical device.

The magnesium fluoride-containing layer 113 is a layer that decreasesthe difference in refractive index between air and the layered structure111 to enhance the antireflection performance, and is a layer thatcontains magnesium fluoride (MgF₂) as a main component. The refractiveindex of the magnesium fluoride-containing layer 113 at d line(wavelength: 587.6 nm) is preferably 1.40 or less and more preferably1.38 or less. The magnesium fluoride content of the magnesiumfluoride-containing layer 113 is preferably 80 mass % or more and morepreferably 90 mass % or more.

As described above, when a magnesium fluoride-containing layer 113 isdirectly formed on the base member 120 that includes the layeredstructure 111 and has a surface formed of a metal oxide, opticalabsorption occurs at the interface between the metal oxide surface andthe magnesium fluoride-containing layer 113, and the performance of theoptical device is degraded. Thus, in the present disclosure, a bufferlayer 112 is provided between the base member 120 and the magnesiumfluoride-containing layer 113 so that the buffer layer 112 is in contactwith both the base member 120 and the magnesium fluoride-containinglayer 113. In other words, the optical device includes, in sequence fromthe substrate 101 side, a metal oxide layer contained in the layeredstructure 111, the buffer layer 112, and the magnesiumfluoride-containing layer 113.

The buffer layer 112 is a samarium oxide-containing layer or anytterbium oxide-containing layer. The samarium oxide-containing layer isa layer that contains samarium oxide (Sm₂O₃) as a main component, andthe samarium oxide content is preferably 80 mass % or more and morepreferably 90 mass % or more. Similarly, the ytterbium oxide-containinglayer is a film that contains ytterbium oxide (Yb₂O₃) as a maincomponent, and the ytterbium oxide content is preferably 80 mass % ormore and more preferably 90 mass % or more.

In order to suppress optical absorption that occurs at the interfacebetween the base member 120 and the magnesium fluoride-containing layer113, the buffer layer 112 must be a continuous film. On the basis of thethin film growth process (nuclei growth process), it is considered that,at a film thickness of 3.0 nm or more, stable nuclei connect to oneanother and form a continuous film. The optical device is generally usedin a visible light region of 350 to 800 nm. The basic unit of thephysical film thickness used in the optical thin film design is λ/4nwhere λ represents the wavelength of the incident light and n representsthe refractive index of the film, and the film thickness that isactually employed is from λ/4n to ten times λ/4n at most. Since therefractive index of the samarium oxide-containing layer and theytterbium oxide-containing layer at the longest wavelength λ=800 nm inthe wavelength region of the visible light is about 1.9 to 2.0, it isconsidered that the film thickness employed as the optical thin film is1.0 μm or less. Thus, the layer thickness of the buffer layer 112 can be3.0 nm or more and 1.0 μm or less.

Although an optical device that includes, in sequence from the substrate101 side, a layered structure 111, a buffer layer 112, and a magnesiumfluoride-containing layer 113 is described with reference to FIG. 1 ,the structure of the optical device is not limited to this. Depending onthe optical performance required of the optical device, a structure thatuses a base member that does not include a layered structure 111 is alsopossible. When the layered structure 111 is omitted and when thesubstrate 101 is formed of a metal oxide such as glass, the surface ofthe base member, that is, the surface of the substrate 101, is formed ofa metal oxide, and thus optical absorption occurs if a magnesiumfluoride-containing layer 113 is directly formed on the substrate 101.Thus, even with the structure that includes a base member 120 that doesnot include the layered structure 111, a samarium oxide-containing layerthat serves as a buffer layer 112 may be formed between the base member120 and the magnesium fluoride-containing layer 113.

It is possible to impart a reflection enhancing function, a filteringfunction, and the like to the optical device of the present disclosurein addition to the antireflection function by adjusting the material andfilm thickness of the layers constituting the layered structure 111.

Deposition System

FIG. 2 is a schematic view of a deposition system 200 suitable formanufacturing the optical device of the present disclosure. Thedeposition system 200 includes a deposition chamber 201, inside of whichis maintained in a vacuum state, and an exhaust system 202 that includesa vacuum pump for evacuating the deposition chamber 201, etc. Thedeposition chamber 201 is equipped with multiple target units 203 in theinside. Although a structure equipped with multiple target units 203 isillustrated in FIG. 2 , the number of target units may be one.

Each of the target units 203 is equipped with a cooling box 205 thatcontains a magnet 204 inside and that can cool the target withcirculating cooling water. The magnet 204 is arranged so that a magneticfield is formed in a direction parallel to the target surface. Thecooling water flowing in the cooling box 205 is adjusted to a desiredtemperature by a chiller not shown in the drawing, and is supplied tothe cooling box 205 at a particular flow rate to keep the surfacetemperature of the target constant. The cooling box 205 is equipped witha backing plate 206 that serves as a cathode electrode, and one oftargets 207 a to 207 d is fixed to the backing plate 206. An anodeelectrode 209 is disposed along the peripheral portion of the backingplate 206 with an insulating material 208 therebetween. The anodeelectrode 209 and the cathode electrode (backing plate) 206 areconnected to a power supply 211 via a switch 210, and are configured tosupply electric power to one of the targets 207 a to 207 d. When thetargets 207 a to 207 d are respectively composed of different materials,electric power can be supplied to the desired one of the targets 207 ato 207 d by switching the switch 210 so that multiple types of films canbe deposited in the same chamber.

The power supply 211 may be a DC power supply. If a high-frequency ACpower supply is used as the power supply 211, a large self bias voltageoccurs in a substrate 216. This self bias voltage accelerates thecations of the target material, and the cations incident on thesubstrate damage the substrate and the films formed on the substrate. Inparticular, the magnesium fluoride layer is susceptible to damage, andoptical absorption in the visible light region occurs easily. In otherwords, when a DC power supply is used as the power supply 211, damage onthe deposited films can be suppressed, and an optical device withfurther decreased optical absorption can be obtained.

The deposition chamber 201 is connected to a load lock chamber 213 via agate valve 212, and an exhaust system 214 different from that of thedeposition chamber 201 is installed in the load lock chamber 213. Asubstrate holder 215 that holds the substrate 216 can change theposition in the deposition chamber 201 and can move between the loadlock chamber 213 and the deposition chamber 201 when operated by amoving mechanism 217. In this manner, the substrate 216 can be carriedin and out without exposing the inside of the deposition chamber 201 tothe atmospheric air. The substrate holder 215 is also equipped with anangle adjusting mechanism and a rotating mechanism so that the relativeangle between the sputter surface of one of the targets 207 a to 207 dand the substrate holding surface of the substrate holder 215 can bechanged. In addition, a shielding plate 218 that blocks particlessputtered from the target is installed between the substrate holder 215and the targets 207 a to 207 d so that deposition does not start untildischarge has stabilized. In order to accurately control the filmthickness, the shielding plate 218 may be openable and closable at highspeeds.

Furthermore, the deposition chamber 201 is configured so that adeposition gas can be supplied through a gas supplying system from asputtering gas introduction port 219 and reactive gas introduction ports220 to 222. An inert gas such as Ar, He, Ne, Kr, or Xe is supplied as asputtering gas from the sputtering gas introduction port 219. Dependingon the type of the target and the material to be deposited, a reactivegas, for example, oxygen (O₂), fluorocarbon gas, hydrogen (H₂), or thelike, can be introduced from the reactive gas introduction ports 220 to222. The flow rate, the purity, and the pressure of each gas can behighly accurately controlled by a mass flow controller and a gaspurifier. When there are many reactive gases to be introduced, morereactive gas introduction ports may be provided.

Next, a deposition method that uses the deposition system 200illustrated in FIG. 2 is described. The targets 207 a to 207 d areselected according to the type of the thin film to be formed. Here, asone example, the case in which niobium (Nb) for depositing Nb₂O₅ is usedin the target 207 a and silicon (Si) for depositing SiO₂ is used in thetarget 207 b is described. Furthermore, magnesium (Mg) for depositingMgF₂ is used in the target 207 c, and samarium (Sm) for depositing Sm₂O₃is used in the target 207 d. In addition, an Ar gas can be introducedfrom the sputtering gas introduction port 219, an O₂ gas can beintroduced from the reactive gas introduction port 220, a fluorocarbongas can be introduced from the reactive gas introduction portion 221,and a H₂ gas can be introduced from the reactive gas introduction port222.

After preliminary preparation, such as installing targets, is completed,the substrate holder 215 is moved into the load lock chamber 213, thegate valve 212 is closed, and the exhaust system 202 is driven toevacuate the deposition chamber 201 to a pressure of about 1×10⁻³ Pa.While the gate valve 212 is in a closed state, the load lock chamber 213is opened, and the substrate 216 is mounted onto the substrate holder215.

The position of the substrate holder 215 is preliminarily adjusted bythe angle adjusting mechanism so that the film thickness distributionwithin a surface of the substrate 216 onto which a film is to be formed(hereinafter this surface is referred to as a “deposition surface”) isconstant. In depositing a magnesium fluoride (MgF₂) layer, the positionmust be adjusted so that the deposition surface of the substrate 216 isoutside the projection plane of the target in a normal line direction ofthe sputter surface and so that the film thickness distribution withinthe surface of the substrate 216 is constant.

When a fluoride film is deposited by reactive sputtering using a typicalparallel plate-type magnetron sputtering device, a thin compound film,such as a magnesium fluoride film, is formed on a target surface due tothe influence of the reactive gas. When the surface (sputter surface) onwhich a fluorine-containing compound film is formed is sputtered,negative ions are partly formed, and the formed negative ions areaccelerated by the ion sheath voltage and turn into negative ions havinglarge kinetic energy and directivity. Since these negative ions areaccelerated in a direction substantially perpendicular to the targetsurface, placing the substrate within the projection plane in the normalline direction of the sputter surface causes the negative ions havinglarge kinetic energy to collide with the substrate, inflicting extensivedamage on the substrate or the film formed on the substrate surface.Thus, the substrate is arranged so that the lines normal to the sputtersurface of the target do not intersect the deposition surface, in otherwords, the substrate is arranged so as not to overlap a region where thetarget is projected in a normal line direction with respect to thesputter surface. In this manner, damage on the film can be suppressedeven when negative ions are formed.

Next, the shielding plate 218 is put in a closed state so that a film isnot deposited on the deposition surface of the substrate 216, and Ar isintroduced into the deposition chamber 201 from the sputtering gasintroduction port 219. Next, the switch 210 is operated to connect thepower supply 211 to the cathode electrode (backing plate) 206 on whichthe target 207 a for forming a high-refractive-index material, niobiumoxide film to be formed first is installed, and a particular DC voltageis applied. At the target 207 a, glow discharge occurs (plasma isgenerated), and Ar turns into cations. These cations collide with thetarget 207 a electrically connected to the cathode electrode 206, andatoms are sputtered off from the target 207 a.

A plasma is stable even when the pressure inside the deposition chamber201 is about several tenths of Pa. The reason why a plasma is generatedat such a low pressure is that electrons undergo cyclotron motion withina plane perpendicular to the magnetic field due to the magnetron effectof the magnet 204 housed in the cooling box 205, and thus the electrondensity near the target 207 a can be increased. Moreover, since themagnetron of the magnet 204 increases the electron density near thetarget 207 a and decreases the electron temperature and electron densitynear the substrate 216, the magnetron also has effects of suppressingentry of the charged particles into the substrate and decreasing thedamage on the film.

Next, an O₂ gas is introduced as a reactive gas for depositing niobiumoxide (Nb₂O₅), which is a high-refractive-index material, into thedeposition chamber 201 through the reactive gas introduction port 220.

The introduced reactive gas oxidizes the surface of the target 207 a,and the surface is readily covered with an insulating material. Thisinsulating material may become charged up, and may undergo dielectricbreakdown due to ions and electrons, possibly resulting in abnormaldischarge. Once abnormal discharge occurs, foreign substances mix intothe film, giving a film having a rough surface. Thus, an AC voltage ofabout several kHz can be superposed on the DC voltage to cancel thecharges and prevent abnormal discharge. If the frequency to besuperposed is excessively increased, a self bias voltage occurs in thesubstrate as described above, causing the cations to enter the substrateand damage the film. However, as long as the frequency superposed is 100kHz or less, the influence of the damage on the film can be confinedwithin an allowable range.

The pressure inside the deposition chamber 201 during deposition ismaintained at 0.1 Pa or more and 3.0 Pa or less by adjusting the valvesand mass flow controllers respectively installed in the exhaust system202, the sputtering gas introduction port 219, and the reactive gasintroduction port 220. A film having a rough surface and a low densityis formed if the pressure is excessively increased, and discharge dropoccurs if the pressure is excessively decreased. After it is confirmedthat the discharge voltage has stabilized, the shielding plate 218 isopened to start deposition. The film thickness is controlled by thedeposition time on the basis of the relationship between the depositiontime and the film thickness studied in advance.

Deposition of a low-refractive-index-material silicon oxide (SiO₂) layerusing the target 207 b can be performed by applying a DC voltage to thetarget 207 b by operating the switch 210, and the process thereafter canbe the same as the deposition using the target 207 a. In depositing SiO₂also, an O₂ gas is introduced as a reactive gas into the depositionchamber 201.

Deposition of a magnesium fluoride (MgF₂) layer using the target 207 ccan be performed by applying a DC voltage to the target 207 c byoperating the switch 210, and the process thereafter can be the same asthe deposition using the target 207 a. However, during the deposition,an O₂ gas and a fluorocarbon gas are introduced as the reactive gases.

Deposition of a samarium oxide (Sm₂O₃) layer using the target 207 d canbe performed by applying a DC voltage to the target 207 d by operatingthe switch 210, and the process thereafter can be the same as thedeposition using the target 207 a. However, during the deposition, an O₂gas and a H₂ gas are introduced as the reactive gases. When H₂ isintroduced, a multilayer film having further low optical absorption canbe stably obtained even when magnesium fluoride is deposited on thesamarium oxide layer. This is presumably because dangling bonds of Smgenerated due to the failure of the reaction between Sm and O react withH to form a stable H-containing Sm₂O₃ film, and generation of danglingbonds, such as bond cleavage, is suppressed even when electrons in theplasma and accelerated high-energy charged particle beams irradiate theinterface.

In the present disclosure, after a number of niobium oxide layers and anumber of silicon oxide layers that correspond to the optical design arealternately stacked, a samarium oxide layer is deposited on a niobiumoxide layer while introducing O₂ and H₂ as reactive gases, and then amagnesium fluoride layer is formed. According to this method, generationof electronic defects (reduction) in the samarium oxide layer duringformation of the magnesium fluoride layer can be prevented, andoccurrence of optical absorption at the interface can be suppressed. Asa result, even when high-rate deposition using charged particles, suchas a sputtering method, is performed, a fluoride-containing multilayerfilm that has low absorption in the visible region can be obtained. Thesame applies to when an ytterbium oxide-containing layer is formed asthe buffer layer 112.

A schematic sectional view of a thus-obtained optical device 100 isshown in FIG. 1 . The optical device 100 includes, in sequence, a basemember constituted by a substrate 101 and a layered structure 111 formedof a metal oxide and disposed on the substrate 101, a buffer layer 112containing samarium oxide, and a magnesium fluoride-containing layer113. The layered structure 111 is constituted by high-refractive-indexlayers, i.e., niobium oxide layers 111 a, and low-refractive-indexlayers, i.e., silicon oxide layers 111 b, stacked in an alternatingmanner, and the uppermost surface of the layered structure 111 is aniobium oxide layer 111 a. As illustrated in FIG. 1 , since a bufferlayer 112 containing samarium oxide is interposed between the magnesiumfluoride-containing layer 113 and the niobium oxide layer 111 aconstituting the uppermost surface of the layered structure 111, theoptical absorption is suppressed, and an optical device having hightransmittance and excellent properties can be obtained. This opticaldevice can exhibit a reflection enhancing function, a filteringfunction, and the like in addition to the antireflection function.

EXAMPLES Examples 1 to 5

A sample having a structure that includes a substrate formed of siliconoxide (SiO₂), and a samarium oxide (Sm₂O₃) layer and a magnesiumfluoride (MgF₂) layer formed in that order on the substrate was preparedby using the deposition system illustrated in FIG. 2 while varying theamount of H₂ introduced during deposition of samarium oxide layer.

A Mg target was used as the target 207 c, a Sm target was used as thetarget 207 d, and a flat SiO₂ substrate was used as the substrate 216.Fluorocarbon “HFC-245fa (1,1,1,3,3-pentafluoropropane: CHF₂CH₂CF₃)”, O₂,and H₂ were used as the reactive gases.

First, a Sm₂O₃ layer was deposited. A washed substrate 216 was installedin the load lock chamber 213, and the chamber was evacuated to 1×10⁻³ Paor less. Upon completion of the evacuation, the substrate 216 wascarried into the deposition chamber 201 by the substrate holder 215 viathe gate valve 212, and was placed in a deposition position inside thedeposition chamber 201. At this stage, the distance between the target207 d and the substrate 216 was about 80 mm. The shielding plate 218 wasclosed, 420 sccm of Ar was introduced through the sputtering gasintroduction port 219, 100 sccm of O₂ was introduced through thereactive gas introduction port 220, and H₂ was introduced through thereactive gas introduction port 222. The amount of introduced H₂ wasvaried within the range of 0 to 100 sccm as indicated in Table, andsamples of Examples 1 to 5 were prepared. The pressure inside thedeposition chamber 201 was about 0.3 to 0.4 Pa.

A 1500 W sputtering power was applied to the cathode electrode (backingplate) 206 on which the target 207 d was installed so as to generate amagnetron plasma on the surface of the target 207 d. At the same time, a5 kHz rectangular voltage that reverses the polarity of the targetsurface was superposed to cancel the charges on the target surface sothat a stable discharge can be maintained.

Discharging was continued for a while, and after the discharge state hadstabilized, the shielding plate 218 was opened to start deposition. Thefilm thickness was controlled to about 100 nm by controlling thedeposition time on the basis of the relationship between the depositiontime and the film thickness studied in advance. Upon reaching thedeposition time, the shielding plate 218 was closed, supply of gases andapplication of power were stopped, and the deposition of a Sm₂O₃ filmwas ended.

Next, a MgF₂ layer was deposited on the Sm₂O₃ layer on the substrate216. First, the switch 210 was operated so that the target to which theDC power was applied was switched from the Sm metal target 207 d to theMg metal target 207 c. The substrate 216 was arranged so as not tooverlap the region where the target 207 c is projected in a normal linedirection with respect to the sputter surface and so that the filmthickness direction within the deposition surface of the substrate 216was constant. In this manner, the influence on the substrate 216 fromthe negative ions generated on the surface of the target 207 c andaccelerated by the cathode voltage can be suppressed. Ar was introducedthrough the sputtering gas introduction port 219 at a rate of 300 sccm,O₂ was introduced through the reactive gas introduction port 220 at 50sccm, and HFC-245fa was introduced through the reactive gas introductionport 221 at 20 sccm. A 3000 W sputtering power was applied to thecathode electrode (backing plate) 206, and, at the same time, a 5 kHzrectangular voltage that reverses the polarity of the surface of thetarget 207 c was superposed to generate a magnetron plasma. Dischargingwas continued for a while, and after the discharge state had stabilized,the shielding plate 218 was opened to start deposition. A film having athickness of about 30 nm was deposited by controlling the depositiontime.

Table indicates the optical absorption at 450 nm for each of the samplesof Examples 1 to 5 that were prepared under the same experimentalconditions except for the flow rate of H₂. The optical absorption wascalculated on the basis of the reflectance and the transmittance of eachsample measured with a commercially available reflectance/transmittancespectrometer. A sample of Comparative Example 1 was prepared as inExample 1 except that the Sm₂O₃ layer was not deposited; however,separation occurred at the interface between the surface of the SiO₂substrate and the MgF₂ layer. Thus, a sample of Comparative Example 2was prepared by depositing a MgF₂ layer under the same conditions as inExample 1 after a Nb₂O₅ layer was deposited to 100 nm on a surface ofthe SiO₂ substrate, and the optical absorption of this sample wascalculated as in other examples. The results of Examples 1 to 5 andComparative Examples 1 and 2 are indicated in Table. Considering theapplicability to optical devices, samples with an optical absorptionless than 0.2% were rated A, samples with an optical absorption 0.2% ormore but less than 1.0% were rated B, and samples with an opticalabsorption of 1.0% or more or that underwent film separation were ratedC.

TABLE Rate of H₂ introduced during Optical deposition of absorption Filmstructure Sm₂O₃ [sccm] [%] Rating Example 1 SiO₂/Sm₂O₃/MgF₂ 0 0.6 BExample 2 SiO₂/Sm₂O₃/MgF₂ 10 0.4 B Example 3 SiO₂/Sm₂O₃/MgF₂ 30 0.2 BExample 4 SiO₂/Sm₂O₃/MgF₂ 50 0.1 A Example 5 SiO₂/Sm₂O₃/MgF₂ 100 0.02 AComparative SiO₂/MgF₂ — — C Example 1 Comparative Nb₂O₅/MgF₂ — 3 CExample 2

The table shows that depending on the flow rate of H₂ introduced duringdeposition of the MgF₂ layer on the Sm₂O₃ layer, the value of opticalabsorption changes. In Example 1 in which H₂ was not introduced, anoptical absorption of about 0.6% occurred, but when deposition wasconducted by introducing H₂, the optical absorption decreased with theincreasing rate of introduction. At a H₂ introduction rate exceeding 50sccm, excellent optical characteristics suitable for optical devices areobtained. At a H₂ introduction rate exceeding 100 sccm, opticalabsorption rarely occurs, and particularly excellent opticalcharacteristics are obtained.

Among five samples indicated in Table, the sample of Example 1 in whichH₂ gas was not introduced during deposition of the Sm₂O₃ layer and asample of Example 5 in which H₂ gas was introduced at a rate of 100 sccmwere analyzed to determine the bonding state at the interface betweenthe Sm₂O₃ layer and the MgF₂ layer by X-ray photoelectron spectroscopy(XPS analysis). Analysis was carried out by etching the samples with Aruntil a position where Mg, F, and Sm were simultaneously detected wasreached, that is, until the interfacial region between the Sm₂O₃ layerand the MgF₂ layer was exposed. The analytical conditions were asfollows.

Analytic method: X-ray photoelectron spectroscopy (XPS)

Analyzer: Quantera SXM (ULVAC PHI)

X-ray used: Al K-α monochromatic X-ray (1487 eV)/25 W/15 kV/100 μmϕ)Neutralizing: both electron gun and argon ion gun were usedDetected spectrum: Survey spectrum(Binding energy BE=0 to 1150 eV/pass energy PE=280 keV)

Narrow Spectrum

(Sm₃d5/2, O1s/PE=112 keV)

FIG. 3 indicates an extract of some portion of binding energy region ina Survey spectrum of the Ar2p3/2 peak of the Sm₂O₃ layer for each ofExamples 1 and 5. Since these layers were deposited by sputtering, theAr2p3/2 peak was observed around a binding energy of 242 eV, and thisconfirmed that the layers contained Ar. In the present disclosure,“around 242 eV” means the range of 242 eV±1.5 eV, and for other bindingenergy also, “around” means the range of ±1.5 eV.

FIG. 4 indicates a narrow spectrum in the depth direction of the Sm₃d5/2peak around the interface between the Sm₂O₃ layer and the MgF₂ layer inExample 5. FIG. 5 indicates a narrow spectrum in the depth direction ofthe Sm₃d5/2 peak in Example 1. In the present disclosure, “around theinterface” means a range that spans from an etching depth at which theSm peak (Sm₃d5/2) was first observed to a depth reached by performingfive cycles of ion etching after reaching the aforementioned etchingdepth when XPS analysis was performed while gradually etching from thesurface of the MgF₂ layer.

Typically, the binding energy of Sm₃d5/2 of Sm₂O₃ peaks around 1083 eV;however, when Sm₂O₃ is reduced, a peak appear on the lower energy sidearound 1073 eV. Presence or absence of the peak was determined on thebasis of the following standard. A second derivative spectrum wasobtained from the observed spectrum, and if a raised portion appeared ata certain position on the negative side at or below the threshold in thesecond derivative spectrum, it was judged that a peak is present at thatposition. In Example 1 in which the H₂ gas was not introduced duringdeposition of the Sm₂O₃ layer, Sm was reduced around the interfacebetween the Sm₂O₃ layer and the MgF₂ layer, and a peak was observed onthe low energy side. However, in Example 5 in which H₂ was introduced ata rate of 100 sccm, the peak resulting from reduction was not observed.These analytical results confirmed that, depending on the rate of the H₂gas introduced during deposition, a layer containing samarium oxidedeposited by introducing the H₂ gas has a high effect of suppressingreduction at the interface even when a magnesium fluoride layer isdeposited thereon.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An optical device manufacturing method comprising: a step of preparing a substrate; and a step of forming, by reactive sputtering using a metal target, an oxide-containing layer on the substrate, wherein, in the step of forming the oxide-containing layer, an oxygen-containing gas and a hydrogen-containing gas are introduced as reactive gases.
 2. The optical device manufacturing method according to claim 1, wherein, in the step of forming the oxide-containing layer, the hydrogen-containing gas is introduced at a rate higher than 50 sccm.
 3. The optical device manufacturing method according to claim 1, wherein in the step of forming the oxide-containing layer, the hydrogen-containing gas is introduced at a rate higher than 100 sccm.
 4. The optical device manufacturing method according to claim 1, wherein the hydrogen-containing gas is introduced at a rate higher than the oxygen-containing gas is.
 5. The optical device manufacturing method according to claim 1, wherein the oxygen-containing gas is O₂.
 6. The optical device manufacturing method according to claim 1, wherein the hydrogen-containing gas is H₂.
 7. The optical device manufacturing method according to claim 1, wherein, in the step of forming the oxide-containing layer, an inert gas is introduced as a sputtering gas.
 8. The optical device manufacturing method according to claim 7, wherein the inert gas is Ar.
 9. The optical device manufacturing method according to claim 7, wherein, in the step of forming the oxide-containing layer, the hydrogen-containing gas is introduced at a rate lower than the inert gas is.
 10. The optical device manufacturing method according to claim 1, wherein the oxide-containing layer has a thickness of 3.0 nm or more and 1.0 μm or less.
 11. The optical device manufacturing method according to claim 1, comprising: a step of forming, by reactive sputtering using another target, another layer on the substrate.
 12. The optical device manufacturing method according to claim 11, wherein the another target is a metal target.
 13. The optical device manufacturing method according to claim 12, wherein the another target is formed of niobium (Nb), silicon (Si) or magnesium (Mg).
 14. The optical device manufacturing method according to claim 11, wherein the another layer is a fluoride-containing layer or an oxide-containing layer.
 15. The optical device manufacturing method according to claim 14, wherein the another layer contains silicon oxide, aluminum oxide, zirconium oxide, tantalum oxide, niobium oxide, titanium oxide, magnesium fluoride or aluminum fluoride.
 16. The optical device manufacturing method according to claim 1, wherein the substrate is formed of calcium fluoride crystals, quartz glass, silicon, glass or a resin.
 17. The optical device manufacturing method according to claim 1, wherein the reactive sputtering is performed in a deposition system, the deposition system comprising: a deposition chamber; a first introduction port to introduce a sputtering gas into the deposition chamber; a second introduction port to introduce the oxygen-containing gas into the deposition chamber; a third introduction port to introduce the hydrogen-containing gas into the deposition chamber; and a plurality of targets respectively composed of different materials and installed in the deposition chamber, wherein the metal target is one of the plurality of targets.
 18. The optical device manufacturing method according to claim 17, wherein one of the plurality of targets is formed of niobium (Nb), silicon (Si) or magnesium (Mg).
 19. The optical device manufacturing method according to claim 17, wherein the deposition system comprises a fourth introduction port to introduce a fluorine-containing gas into the deposition chamber.
 20. The optical device manufacturing method according to claim 19, wherein the fluorine-containing gas is a fluorocarbon gas. 